Even though the instances of OverTheTop are fixed and immutable, enums are after all simply class instances and can be use anyway a class can be used. This is the same problem with unmodifiable collections having modifiable members.

You could view enums as a generalization of singletons, having a fixed number of members not limited to just one. (Class loading is thread safe — does this mean one can use a single-member enum to implement a thread-safe singleton?)